Drip irrigation conduits receive water under pressure and discharge the water at a slow trickle or drip rate. In order to do this, the irrigation conduit must substantially reduce the pressure of the water. Generally, pressure reduction is provided by using controlled orifices as shown, by way of example, in Chapin U.S. Re. Pat. No. 28,095 and Mock U.S. Pat. No. 3,903,929 or by using an elongated capillary tube having a capillary passage of very small cross-sectional area as shown, by way of example, in allowed Menzel et al U.S. Pat. No. 4,269,357.
Regardless of the manner in which pressure reduction is accomplished, drip irrigation conduits typically include an elongated supply tube having a supply passage and one or more secondary tubes extending along the supply tube. Transfer ports extend between the supply passage and the passage of the secondary tube, and discharge ports extend from the passage of the secondary tube to the exterior of the irrigation conduit or to one or more additional secondary tubes from which the water is ultimately discharged to the exterior of the irrigation conduit at the desired trickle rate.
Drip irrigation conduits of this type can be extruded as shown by Mock U.S. Pat. No. 3,903,929 or the secondary tube and a section of sheet material can be extruded as shown in the Menzel et al patent. In either event, the extrusion of the secondary tube in association with the supply tube or in association with sheet material to form the primary tube has been found somewhat difficult to control, particularly when the secondary tube is to be a capillary tube having a very small cross-sectional area capillary passage. In addition, extrusion of the supply tube precludes the use of inexpensive film materials and paper for the supply tube.
Allport U.S. Pat. No. 4,247,051 discloses a drip irrigation conduit in which a section of sheet material is joined together along longitudinally extending regions by circumferentially spaced and radially thick ribs of adhesive. The sheet material forms the supply passage, and the ribs and sheet material cooperate to define the secondary passage. The deposition of two or more relatively thick adhesive ribs of constant thickness and the careful pressurizing of the thick adhesive ribs between overlapping regions of the sheet material to form a secondary passage of controlled cross-sectional area is believed to be very difficult to accomplish in production.